This invention relates generally to medical imaging systems, and more particularly to attenuation correction for medical imaging.
A nodule found during a computed tomography (CT) scan often requires a patient to return many months later and obtain another CT scan to determine malignancy based on a nodule doubling time. Positron emission tomography (PET) scans may be helpful in diagnosis due to increased metabolic activity in the region of the nodule. However, due to the comparatively lower resolution of PET images as compared to CT images, and due to the effects of respiratory or patient motion during a PET scan, nodule activity can be blurred in the PET scan. Consequently, it can be difficult to quantify the nodule activity with a PET scan alone, which may result in an indeterminate or incorrect outcome of the diagnosis of the nodule.
More particularly, the image quality of at least some known PET and CT systems is highly affected by physiological patient movement. Such image quality may affect diagnosis. Lung nodules, cardiac wall features or other small features of interest that move due to physiological motion such as cardiac and respiratory motion may appear blurred or even absent without proper corrections. Therefore, attenuation correction is performed, where an attenuation correction map derived from CT transmission images is used to correct the PET images. Further, misalignment of a CT attenuation map and the PET emission image that is due to respiratory motion may cause errors in attenuation correction (AC) factors and may produce artifacts in the final reconstructed AC PET image. For instance, “under attenuation correction” may have the potential of introducing artifacts that resemble artificial myocardial perfusion defects in cardiac PET. Thus, in the case of cardiac PET, wherein helical CT data is used, attenuation artifacts may result in areas having artificially reduced tracer uptake in the myocardial wall that may be incorrectly interpreted as perfusion defects by utilizing helical CT attenuation correction (CTAC) data.